The present invention relates to microprocessor based electro-pneumatic type locomotive brake control systems and particularly to a pneumatic backup brake control circuit for such a locomotive brake control system.
Modern-day locomotive controls, including the locomotive brake control system, incorporate computer technology to reduce hardware and to facilitate adaptation of the system to various customer requirements.
In one such brake control system, disclosed in U.S. Pat. No. 5,192,118, issued Mar. 9, 1993, and incorporated herein by reference, a cab-mounted, handle-operated, brake controller outputs a desired brake command signal to a microprocessor unit, which interprets this brake command signal in terms of a feedback signal indicative of the pressure of air in an equalizing reservoir, and then effects operation of application and release electro-magnetic valves to adjust the equalizing reservoir pressure in accordance with the brake command signal.
A high-capacity pneumatic relay valve device is employed to vary the trainline brake pipe pressure in accordance with variations of the equalizing reservoir pressure, in order to control the railway car brakes. This so-called brake pipe control circuit of the aforementioned brake control system is shown and described in U.S. Pat. No. 4,904,027.
The brake control system further includes a locomotive brake cylinder control circuit having electropneumatic application and release valves. These electropneumatic valves are operated by the microprocessor in response to changes in brake pipe pressure initiated by the brake pipe control circuit in accordance with movement of an automatic brake handle of the cab brake controller. Another high-capacity pneumatic relay valve device regulates the pressure in the locomotive brake cylinders according to the pressure output of the locomotive brake cylinder control circuit application and release electropneumatic valves.
The electropneumatic valves in the brake pipe control circuit and in the locomotive brake cylinder control circuit are arranged to assume a pressure release state, in the event of a power loss at the microprocessor unit. In consequence of such a power loss, therefore, brake pipe pressure is reduced while, concurrently, the locomotive brake cylinder pressure is released. A pneumatic back-up control valve is provided to establish locomotive brake cylinder pressure in response to the afore-mentioned reduction of brake pipe pressure resulting from such fail-safe operation of the electro-pneumatic valves in the brake pipe control circuit, there being a double check valve to separate the pneumatic backup control valve from the electro-pneumatic valves in the locomotive brake cylinder control circuit.
The pneumatic backup control valve includes a piston valve assembly subject on opposite sides to compressed air in the brake pipe and in a supply reservoir. When brake pipe pressure is reduced, the resultant pressure differential forces the piston valve assembly to application position, wherein the supply reservoir air supplies the brake cylinder pilot line to establish the locomotive brake pressure until a force balance is restored across the piston valve assembly. In this manner, the piston valve assembly seeks a lap position in which the supply of brake cylinder pressure is terminated at a value corresponding to the brake pipe pressure reduction in effect.
This locomotive brake cylinder pressure may be released independently of the car brakes by means of a quick release valve associated with the pneumatic backup control valve. A pressure signal supplied to the quick release valve, when a quick release switch is actuated, initiates this "bail-off" or quick release function. The brake pipe supply reservoir pressures are communicated via the quick release valve to establish pressure equalization across the piston valve assembly, when the quick release switch is actuated. In this manner, the supply reservoir pressure is effectively equalized with the reduced brake pipe pressure, such that spring force acting on the piston valve assembly is effective to force the piston valve assembly to release position and the locomotive brake cylinder pressure is exhausted.
In a similar manner, a dynamic brake interlock magnet valve is employed to actuate the quick release valve and accordingly maintain the locomotive brake cylinder pressure exhausted so long as the dynamic brake effectiveness is sufficient to hold in the magnet valve. The purpose of this is to prevent excessive brake forces and accordingly a wheel slide condition from occurring. Such an arrangement has proven to work reliably under most conditions. Where high operating pressures are required, however, a relatively large supply reservoir is employed, the volume of this reservoir being capable of maintaining a pressure head on the supply reservoir side of the backup control valve piston valve assembly, even when brake pipe pressure on the opposite side is released at an emergency rate. It will be appreciated therefore that prior to the dynamic brake being knocked out in response to a train line-initiated emergency application, for example, the supply reservoir pressure is unable to decrease at the same rate as the brake pipe pressure and a pressure differential is created across the piston valve assembly in a brake application direction. An application of the locomotive air brakes through the pneumatic backup valve can thus occur while the dynamic brake is still effective to create the above-mentioned undesirable wheel slide condition. It will be understood that while interlock circuits are provided to knock out the dynamic brakes under such prevailing train line-initiated emergency conditions, typically there is an inherent delay in this action taking effect, during which time, although relatively brief, the above-discussed condition can arise.